This invention relates to display devices including an electro-optical or electrolytic display medium and, more particularly, to display devices in which the thickness of transparent electrodes and/or transparent insulating films formed on a transparent substrate is optimized.
Recently, display devices wherein a display medium is enclosed between substrates have been used in office automation equipment. These display devices include liquid crystal display devices and electrochromic display devices. In such devices at least one substrate is transparent and has a transparent electrode disposed thereon in order to permit observation of the display medium through the transparent electrode. When the display capacity is increased in these devices, the degree of drive multiplexing is increased and the contrast ratio decreases. The decrease in contrast ratio occurs as a result of reflection between the transparent substrate and the transparent electrode provided on the substrate or between the transparent electrode and the liquid crystal display medium in the case of a liquid crystal display device. When reflections occur, light other than that necessary for display is observed and observation of the display becomes difficult.
Furthermore, when an insulating film is provided on the electrode for protecting the electrode, for preventing application of DC voltage to the liquid crystal layer, and for improving the performance of the display device through control of the orientation of the liquid crystal molecules, additional reflected light is generated. For example, a display device of the twisted nematic type was produced using glass substrates. An indium oxide-tin oxide (ITO) film having a thickness of 800 .ANG. was used as the transparent electrode and a silicon dioxide (SiO.sub.2) insulating film having a thickness of 1000 .ANG. was formed on the electrode. A liquid crystal display cell was formed by enclosing a liquid crystal material between two electrodes. A polarizing plate was attached to one side of the display and a second polarizing plate and irregular reflecting plate was attached to the opposite side of the display. The linear reflectivity of the display device was about 8%, of which about 4% resulted from reflection from the internal surfaces of the liquid crystal display cell. Since the total reflectivity of the device was about 23%, it is clear that the amount of reflected light that was unnecessary for display was equal to about one-third (1/3) of the total reflectivity; only about two-thirds (2/3) of the total amount of reflected light was necessary for display. The reflected light from the surface boundary of the transparent electrode deteriorated the quality of display of the device. These disadvantages impose a serious constraint of the use and popularity of these devices.
It is, therefore, desirable to provide a display device that overcomes the disadvantages inherent in prior art display devices.